1. Field of the Invention
The present invention relates generally to testing of printed circuit boards for use in electronic products, such as computers, and the like. More specifically, a hold down gate is provided which uses a fluid actuated piston to firmly dispose a board being tested adjacent a test apparatus.
2. Description of Related Art
Apparatus for testing printed circuit boards and cards are well known in the art. For example the Hewlett-Packard SMT-Series 3065 and 3070 are commercially available and widely used in the industry. Most testers utilize a "bed-of-nails" design, which includes a probe surface having plural (thousands) of sockets that are interconnected to test equipment, such as a computer with the appropriate software. Test probes are insertable in these sockets and protrude upwardly from the probe surface. These probes are configured to match the input/output connection points of the electronic components, such as integrated circuits (ICs), resident on the circuit board being tested. Further, the probes are biased upwardly such that to ensure proper alignment, a card must be placed over the probes and sufficient downward force must be provided to such that proper electrical connection is made between the I/Os of the electronic components and the test equipment, via the biased test probes.
Conventional test apparatus utilize a vacuum system to provide the downward force necessary to properly seat the card, under test, over the probes. A frame and resilient seal are provided, the seal being disposed around the perimeter of the frame such that a cavity is formed by the underside of the card, the top side of the probe surface and the seal. This cavity is then evacuated, thereby forcing the card downwardly on to the test probes. Several problems exist by using the configuration of conventional test apparatus. For example, maintaining the seal such that sufficient force is provided to ensure proper contact between the card and probes requires frequent maintenance and costly down time often caused by the introduction of contaminants such as dust, bits of substrate material, grease, or the like. Additionally, each test probe requires approximately one half pound per square inch (0.5 lbs/in2) of force to ensure proper electrical connection. The high density printed circuit boards utilized by today's computers often have numerous electronic components and corresponding I/Os such that as many as 2,000, or more test probes are needed. Therefore, it can be seen that 1,000 pounds, or more of downward force may be required to properly seat the board being tested on the probes. These force requirements are not adequately and reliably met by utilization of a vacuum system of conventional test apparatus.
Vacuum systems for holding printed circuit boards adjacent a bed-of-nails type test apparatus are shown by U.S. Pat. Nos. 4,636,723; 4,771,234; 4,164,704; 4,321,533; 4,344,033; and 4,017,793, which also shows the utilization of positive air pressure acting against the PCB to force it into contact with the test equipment.
Further, U.S. Pat. Nos. 4,357,062 and 4,099,120 show expandable air bags for providing contact between test equipment and the components on printed circuit boards.
Threaded rods driven by a belt and stepper motor arrangement are used by U.S. Pat. Nos. 4,993,136 and 4,818,933 for biasing an upper probe plate against the surface of a printed circuit board being tested.
A piston assembly is used in U.S. Pat. No. 4,962,356 to apply uniform and controllable pressure to a single integrated circuit device. The pressure is applied to a carrier and in a direction normal to the IC, such that bending test socket pins and gouging IC leads is prevented.
U.S. Pat. No. 4,357,575 uses an external piston assembly to move a head and contact assembly between a standby and test positions for test of printed circuit boards.
U.S. Pat. No. 4,812,754 employs pistons in communication with a locking plate and platens to draw test heads into contact with a PCB. when the pistons are actuated.
Thus, it would be desirable to have a hold down, or gate which could reliably provide the force necessary to ensure proper connection of the electronic components of the board with the test equipment, and eliminate the costly down time associated with maintenance of a vacuum system, particularly the of resilient seals. Further, problems associated with fluid pressure bags, or diaphragms and rotating threaded rods will also be eliminated by the system of the present invention. Additionally, a much simpler and reliable piston arrangement is shown by the present invention for testing entire printed circuit boards and providing the required force to create and maintain proper electrical contact than is utilized by conventional PCB hold down techniques using pistons.